What is Washington, D.C.’s weather like in May? Breaking down norms and extremes

Flowers and momentos at the Vietnam War Memorial near Memorial Day. By CWG photographer Kevin Ambrose.

May is also the final month of meteorological spring (Mar.-May), and by the end of it -- when 80-degree days are the norm -- summer’s presence is usually widely known.

Keep reading for more in this secondinstallment of a year-long series of posts detailing specific statistics on climate norms, extremes, and averages for the each month in Washington, D.C…

PRECIPITATION

May’s average rainfall is 3.99” (previously 3.82” from 1971-2000)*. As noted, this ranks first among the months at D.C. Whereas April jumped several spots in the new 1981-2010 averages, May retained its previous rank as number 1.

May rainfall, in inches, at Washington, D.C. since records began. (See sources for data)

The maximum daily rainfall in May is 3.24” and it fell on the 5th in 1953. One other day, all the way back in 1900 also saw a 3”+ rainfall. In total, there have been 17 occurrences of daily totals over 2”. Compared to April, average days with at least a trace of rain are slightly higher, 15.6 vs. 15.2. Disruptive rains of .1” or more are also a bit more frequent with 7.3 compared to 6.5 in April.

Change in .1” rainfall days as an average of each 30-year period since the beginning of records at Washington, D.C. (See sources for data)

TEMPERATURES

May temperature averages at Washington, D.C. since records began. Red line is the 1971-2000 temperature average provided by the NWS. (See sources for data)

Highs are typically in the 70s and 80s, 41% and 29% of the time respectively. The coldest high temperature in May was 46 on the 5th in 1917. The warmest high temperature was 99 in 1991.

Nighttime lows are on the way up too, with lows mainly in the 50s (51%) and 60s (31%). There have been no freezes at D.C. past April until the next cold season. May’s coldest temperature fell just short of freezing back on May 11, 1906 with a reading of 33. More recently, the coldest DCA has seen was 34 in 1947. The warmest low temperature in May was 78 on the 30th in 1991.

Number of occurrences of a high temperature equalling 80 degrees or greater on each day in May during the 30-year period from 1981-2010. The 80-degree average arrives on the 30th. (See sources for data)

Though 90+ in April is not super uncommon, May is the typical start of the 90-degree day season at D.C., coming in at 1.6 days per year out of the broader average of 36.4. 1991 was the clear standout in this regard, with a week-long stretch of 90+ to finish the month catapulting it to the warmest May on record.

*All averages, unless otherwise noted, refer to the 30-year period of 1981-2010.

**All instances of monthly temperature average, daily temperature average or daily precipitation average refer to the 1971-2000 climate normals as presented by the National Weather Service. NWS smooths some averages without daily data in a method described here. Monthly rainfall is a simple average and new 1981-2010 data is used here. NWS data on temperature averages should update shortly and will be reflected in these posts.

ABOUT THIS SERIES

While numerous local climatology statistics including daily, monthly, and seasonal normals and records are available from National Weather Service Baltimore/Washington, there are myriad other pieces of information which help frame the story. Rather than attempt to recreate what is already available, this series of posts will act as a companion to most information presented there.

This is the second installment of a series of living documents. Through early 2012, each month will be broken down in a similar or evolving way. As input is taken from the community, some items may be added to or changed in older postings. These postings will be updated as necessary to reflect changes when needed. Other articles may also become part of this series.

Feel free to share thoughts or additional data you may be interested in seeing.